JOYS+: Mid-infrared detection of gas-phase SO₂ emission in a low-mass protostar

The case of NGC 1333 IRAS 2A: Hot core or accretion shock?

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands
e-mail: vgelder@strw.leidenuniv.nl
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³ Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁵ Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory, 439 92 Onsala, Sweden
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ SETI Institute 189 Bernardo Avenue, 2nd Floor, Mountain View, CA 94043, USA
⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁹ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁰ NASA Ames Research Center, Space Science and Astrobiology Division M.S. 245-6 Moffett Field, CA 94035, USA
¹¹ Department of Experimental Physics, Maynooth University, Maynooth, Co Kildare, Ireland
¹² UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹³ Bay Area Environmental Research Institute and NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁴ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ School of Earth and Planetary Sciences, National Institute of Science Education and Research, Jatni 752050, Odisha, India
¹⁶ Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India

Received: 29 September 2023
Accepted: 24 November 2023

Abstract

Context. Thanks to the Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST), our ability to observe the star formation process in the infrared has greatly improved. Due to its unprecedented spatial and spectral resolution and sensitivity in the mid-infrared, JWST/MIRI can see through highly extincted protostellar envelopes and probe the warm inner regions. An abundant molecule in these warm inner regions is SO₂, which is a common tracer of both outflow and accretion shocks as well as hot core chemistry.

Aims. This paper presents the first mid-infrared detection of gaseous SO₂ emission in an embedded low-mass protostellar system rich in complex molecules and aims to determine the physical origin of the SO₂ emission.

Methods. JWST/MIRI observations taken with the Medium Resolution Spectrometer (MRS) of the low-mass protostellar binary NGC 1333 IRAS 2A in the JWST Observations of Young protoStars (JOYS+) program are presented. The observations reveal emission from the SO₂ v₃ asymmetric stretching mode at 7.35 µm. Using simple slab models and assuming local thermodynamic equilibrium (LTE), we derived the rotational temperature and total number of SO₂ molecules. We then compared the results to those derived from high-angular-resolution SO₂ data on the same scales (~50–100 au) obtained with the Atacama Large Millimeter/submillimeter Array (ALMA).

Results. The SO₂ emission from the v₃ band is predominantly located on ~50–100 au scales around the mid-infrared continuum peak of the main component of the binary, IRAS 2A1. A rotational temperature of 92 ± 8 K is derived from the v₃ lines. This is in good agreement with the rotational temperature derived from pure rotational lines in the vibrational ground state (i.e., v = 0) with ALMA (104 ± 5 K), which are extended over similar scales. However, the emission of the v₃ lines in the MIRI-MRS spectrum is not in LTE given that the total number of molecules predicted by a LTE model is found to be a factor of 2 × 10⁴ higher than what is derived for the v = 0 state from the ALMA data. This difference can be explained by a vibrational temperature that is ~100 K higher than the derived rotational temperature of the v = 0 state: T_vib ~ 200 K versus T_rot = 104 ± 5 K. The brightness temperature derived from the continuum around the v₃ band (~7.35 µm) of SO₂ is ~180 K, which confirms that the v₃ = 1 level is not collisionally populated but rather infrared-pumped by scattered radiation. This is also consistent with the non-detection of the v₂ bending mode at 18–20 µm. The similar rotational temperature derived from the MIRI-MRS and ALMA data implies that they are in fact tracing the same molecular gas. The inferred abundance of SO₂ , determined using the LTE fit to the lines of the vibrational ground state in the ALMA data, is 1.0 ± 0.3 × 10⁻⁸ with respect to H₂, which is on the lower side compared to interstellar and cometary ices (10⁻⁸−10⁻⁷).

Conclusions. Given the rotational temperature, the extent of the emission (~100 au in radius), and the narrow line widths in the ALMA data (~3.5 km s⁻¹), the SO₂ in IRAS 2A likely originates from ice sublimation in the central hot core around the protostar rather than from an accretion shock at the disk–envelope boundary. Furthermore, this paper shows the importance of radiative pumping and of combining JWST observations with those from millimeter interferometers such as ALMA to probe the physics on disk scales and to infer molecular abundances.